I play almost every single-player game now on easy, with a few exceptions. I dont have time to play that often anymore, maybe an hour or two a day at best. So especially for the long games, like Witcher 2 and Dragon Age Origins, I tone down the difficulty so I could see and experience everything without having to pour my life into the game. I do the same for Western Action RPGs and Action Games.
I tone down FPS games (Killzone, Resistance, Borderlands, COD, BF) primarily because I suck at FPS in the first place (with the exception of the Valve games that I willed myself to finish on Normal).
The several exceptions to the rule are varied though. When I play the non-campaign portions of RTS, I keep it at normal and ramp it up when I feel I can. I apply this to Civilization, Age of Empires, Dawn of War and Total War. I go through the campaigns on easy so I could check the difficulty. Japanese RPGs (Final Fantasy, Dragon Quest, etc) I play on normal because if you play with proper planning, it is relatively easy.
When I play co-op games, I stick to normal. Primarily because I want to hide the inherent shame of admitting that you play on easy. Indie games (Terraria, Magika, Amnesia) I play on normal.